1. Department of Child Health, St. George's Hospital Medical School, Cranmer Terrace, London SW17 ORE, U.K.
2. Departments of Medical Pharmacology and Neurology, Uppsala University, 5-75123 Uppsala, Sweden.
3. MRC Human Biochemical Genetics Unit, Wolfson House, 4 Stephenson Way, London NW1 2HE, U.K.